Parkinson's disease (PD) is a neurodegenerative disorder characterized by muscle rigidity, tremor, a slowing of physical movement (bradykinesia) and, in extreme cases, a loss of physical movement. The primary symptoms are the results of decreased stimulation of the motor cortex arising from the basal ganglia normally caused by the insufficient formation and action of dopamine, which is produced in the dopaminergic neurons of the brain. Secondary symptoms may include high level cognitive dysfunction and subtle language problems. Included in the symptomatology experienced by patients with PD, visual abnormalities are not uncommon. Visual changes among patients with PD appear not only dynamic in nature, but differentially affected based on the course of the disease and, perhaps more importantly, its treatment. Parkinson's disease has significant ramifications not only in observation of irregularities in vision, but how vision interacts with entrainment of the circadian clock. The purpose of this study is to examine the relationship between PD and operation of a unique set of retinal cells known to regulate the circadian clock and sleep-wake cycles in human subjects.